Some related background is set forth in U.S. Pat. No. 4,135,885, "Burning and Desulfurizing Coal," the contents of which are herein incorporated by reference. Further related background and disclosures are set forth in the application of Alex F. Wormser, "Fluid Bed Combustion," Ser. No. 31,782, filed Apr. 20, 1979 in the United States PTO.* FNT *Contents hereby incorporated by reference herein.
Efforts to burn coal in fluid bed combustors have met with problems both in turndown (burning at less than maximum capacity) and startup preheat, as a number of those in the art have recognized. One approach to turndown has been to use a multiplicity of combustor units and to totally shut down one or more of them. Another approach has been simply to turn a single combustor off and on fairly frequently. Such approaches have required a good bit of hardware, or been clumsy, or made provisions for very little modulation between full-burning and non-burning conditions, or been characterized by an undesirably low turndown ratio, or made impractical turnoff except at the cost of time-consuming restarts, or several of these. The prior art has also taught away from using excess air for the purpose of bed cooling during turndown.
Heated storage zones have been suggested for use in connection with turndown. One variety of storage bed is disclosed in the above-mentioned U.S. patent. (This patent is not in the prior art.) A storage bed generally similar is disclosed in this latter disclosure, such a storage bed is said not to be satisfactory.